Method and apparatus for remotely determining antenna input impedance

ABSTRACT

A system for remotely measuring antenna input impedance utilizing an automatic antenna tuner in which for any tuner, the tuner input impedance may be established by the values of the components in the circuit at the time that the SWR is at a minimum, with these values being automatically available in terms of the components that are switched in to achieve low SWR. Once having established the input impedance to the tuner, the complex conjugate of the tuner input impedance yields the antenna input impedance, with antenna input impedance sensed at full power at a position remote from the antenna input.

FIELD OF INVENTION

This invention relates to the measurement of antenna impedance and moreparticularly to remotely sensing antenna impedance at the antenna feedpoint utilizing an automatic antenna tuner.

BACKGROUND OF THE INVENTION

It has long been a problem to measure antenna impedance at the feedpoint of an antenna when the antenna has been installed or is in placeand is remote from the transmitter to which is coupled. In order to beable to transmit efficiently, it is important to be able to match theoutput impedance of a transmitter to the input impedance of the antennaand to do so antenna tuners are interposed between the feed point of theantenna and the transmitter output. Antennas typically have atransmission line that connects them to the output of the transmitterand the antenna tuners only function to sense impedance at thetransmitter end of the transmission line and match that impedance to theoutput impedance of the transmitter.

While this couples maximum power from the transmitter to the transmitterend of the transmission line, it does not address any mismatch betweentransmitter output impedance and antenna input impedance. In operationof the radio station, whether it is a commercial radio station oramateur radio station, oftentimes environmental factors affect theantenna impedance. For instance, snow, rain, wind and moisture canaffect the antenna input impedance which causes it to vary from itsdesign parameters. Also corrosion and wear can alter the input impedanceof the antenna such that the antenna loses its efficiency. However thisloss of efficiency is not detectable at the transmitter, which istypically remote from the antenna, and is coupled to the antenna by alength of transmission line. Thus technicians at the transmitter have noway of knowing the actual condition of the antenna and cannot, forinstance, be made aware of changes in the antenna input impedance. Theresult is that a transmitting station originally set up and optimizedmay not be operating in an optimal fashion, with this fact not beingknown to the station operator. The following details how, in the past,station engineers have remotely sensed the condition of their antennas:

Typically station engineers have utilized antenna analyzers which couplea low-level signal, in the milliwatt range, to the antenna feed point ata predetermined frequency, with the standing wave ratio (SWR) measuredby the antenna analyzer. However due to the relatively low output ofportable antenna analyzers, nearby radio stations affect the reading ofthe antenna analyzer. On some occasions it is difficult to obtainreliable readings from the antenna analyzer due to high power RF signalsin the area which tend to swamp out the relatively low antenna analyzeroutput signals.

Sources for these RF signals include commercial radio stations, andmobile radios that serve for instance as taxi radios, truck radios, busand commercial vehicle communications. Also high power signalsgenerated, for instance, by power lines, electrical transformers, localradars and even transponders operating in the area can swamp out theantenna analyzer signals.

In order to measure antenna impedance of the antenna feed point,typically one would have to disconnect any antenna tuner utilized and inone method connect a 50 ohm resistor at the transmitter side of theantenna tuner connected to the antenna. Having disconnected the antenna,one then would connect the above-mentioned antenna analyzer to theantenna side of the antenna tuner and would measure the input impedanceof the antenna utilizing the antenna analyzer connected to thealready-tuned antenna tuner. The complex conjugate of the measuredimpedance at the antenna analyzer is the antenna feed point impedance.

The above procedure is quite complicated and first and foremost involvesremoving the antenna from the tuner and second involves using alow-power antenna analyzer which is not the same as using actualoperating conditions

A second way to measure antenna input impedance is to interpose anantenna tuner between the antenna and the transmitter and then utilizethe antenna tuner to tune the transmitter output to the impedance at theend of the coaxial cable coupled to the antenna tuner. Thereafter onedisconnects the transmitter and the antenna and installs the antennaanalyzer on the transmitter side of the antenna tuner while at the sametime substituting a variable resistor, a variable capacitor, and/or avariable inductor, in series, for the removed antenna at the antennainput to the tuner. By changing all of the values of these variableelements; one then adjusts all of such values until one achieves an SWRof 1:1. When this is achieved, one determines the value of theresistance and capacitance and/or inductance to determine the feed pointimpedance of the antenna.

This system of measuring antenna input impedance is likewise cumbersome.

There is therefore a need for a simplified method to remotely measureantenna input impedance and to do so at that end of the transmissionline which is coupled to the transmitter, with the measurement beingmade at full transmitter power so that the measurement is not swamped bylocal signals and is made under actual operating conditions.

It is noted that in some instances, if impedance is not measured at fullpower, there can be anomalies in the measurement when measuring at a lowpower and then increasing power. Typically, a 1:1 SWR at low power maychange when power is increased.

SUMMARY OF THE INVENTION

In order to remotely measure instantaneous antenna input impedance, anautomatic antenna tuner is inserted between the transmitter and thetransmission line to the antenna. Since every tuner can be characterizedas having capacitive and inductive components, the values of thesecomponents when the tuner achieves a minimum SWR defines the impedanceof the antenna tuner. Knowing the impedance of the antenna tuner, thecomplex conjugate of the antenna tuner impedance is the antenna inputimpedance corrected for transmission line length and velocity factor.

Establishing the impedance of the antenna tuner is simplified whenutilizing automatic antenna tuners because when the antenna tunerachieves a minimum SWR, certain components will be switched into thetuner circuit. Knowing the values of the switched-in components byknowing the switching state at the time of minimum SWR provides aconvenient way of finding the impedance of the antenna tuner at tune.The impedance of the antenna input being the complex conjugate of theantenna tuner impedance provides a direct remote readout of the antennainput impedance when one takes into account the length of thetransmission line and its velocity factor. Note that if one canestablish the antenna tuner input impedance at the time of tune themanner in which the tuner obtains a minimum SWR is unimportant, as longas one can ascertain the values of the components which establish tunedstate. Knowing the input impedance to the tuner immediately provides theantenna input or feed point impedance as the complex conjugate of thetuner input impedance. Thereafter, known techniques having to do withthe length of the coaxial cable and its velocity factor are used toremotely detect the antenna's input impedance without having to removethe antenna from the tuner and wherein the remote antenna impedancemeasurement is done at full power.

For instance, at a given frequency the antenna tuner is made to tune andthe inductance and capacitance values of the antenna tuner when a 1:1SWR is achieved are utilized to directly calculate the antenna inputimpedance given the length of the transmission line and its velocityfactor.

In a typical L tuner, in one embodiment, if the load is less than 50ohms, the capacitor is switched to the transmitter side of the inductorand antenna input impedance, as calculated, is given by the followingformula and referring to FIG. 6A:

Antenna input impedance Z _(ant) =R _(in) −j X _(in) where X _(in) =X_(L)−[(R ² X _(C)/(R ² +X _(C) ²)]

Here, R_(in) is the resistive component of the antenna input impedance,whereas X_(in) is the reactive component of the antenna input impedance.

For this type of antenna tuner, it can be shown that Rin=RX_(c)²/(R²+X_(c) ²) when the load is less than 50 ohms and the capacitor isto the transmitter side of the inductor.

Alternatively, referring to FIG. 6B, when the load is greater than 50ohms and the capacitor is to the antenna side of the inductor,R_(in)=RX_(c) ²/[R²+(X_(L)−X_(c))²]

Note that the value of the reactive impedance at the tuner, X_(in), willbe different for the two cases of the load being greater than or lessthan 50 ohms. Specifically, the reactive impedance would be defined as:X_(in)=−X_(C)[X_(L)(X_(L)−X_(C))+R²]/[R²+(X_(L)−X_(C))²].

For any tuner, the tuner input impedance may be established by thevalues of the components in the circuit at the time that the SWR is at aminimum, with these values being automatically available in terms of thecomponents that are switched in to achieve low SWR. Once havingestablished the input impedance to the tuner, the complex conjugate ofthe tuner input impedance yields the antenna input impedance, sensed atthe tuner.

Note that X_(C) is the capacitive reactance of the tuner when tuned andX_(L) is the inductive reactance of the tuner when tuned, convenientlyavailable from the switch states of the automatic tuner. It will beappreciated if a manual tuner is used and one can ascertain the value ofcapacitance and inductance, one could remotely calculate the antennainput impedance as these manually produced values at tune establish theinput impedance of the antenna tuner and thus the complex conjugatespecifies the antenna input impedance.

In one type of tuner, if the load is greater than 50 ohms, the capacitoris placed to the antenna side of the inductor and the antenna tunertunes in this configuration. The antenna tuner switches the capacitorback and forth from one side of the inductor to the other and then usesthe values associated with the lowest SWR to calculate antenna inputimpedance.

Having derived the impedance of the tuner when the tuner is tuned, itsoutput may be corrected for transmission line length and velocity factoras described in a program published by the ARRL and entitledTransmission Line for Windows, and in other available publications ormay be manually calculated.

Having described an automatic antenna tuner characterized by an LCcircuit, the result is that by simply viewing a display on the automaticantenna tuner one can read out the actual instantaneous antenna inputimpedance without having to physically go up to the antenna. With theautomatic antenna tuner, not only is the antenna tuned in a traditionalsense, any antenna malfunction or lack of efficiency due to a change inantenna input impedance is readily observable. Moreover, allmeasurements are made at full power since the use of a low power antennaanalyzer is eliminated. This provides an accurate measurement of antennaperformance under actual operating conditions and obviates thecumbersome procedures noted above.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the subject invention will be betterunderstood in connection with the Detailed Description in conjunctionwith the Drawings of which:

FIG. 1 is a diagrammatic illustration of a prior art solution forremotely determining the input impedance of an antenna utilizing anantenna analyzer coupled to a tuner to which is coupled an RC and/or RLcircuit in which the capacitive and/or inductive and resistive elementsare variable to achieve an SWR of 1:1, with the values of these elementsdetermining the input impedance to the antenna;

FIG. 2 is a diagrammatic illustration of a prior art solution forremotely determining the input impedance of an antenna utilizing anantenna analyzer coupled to the antenna input terminals of a tuner,having a 50 ohm resistor coupled across the tuner at the end to beconnected to the transmitter, with the complex conjugate of the tunerimpedance equaling the antenna input impedance;

FIG. 3 is a diagrammatic illustration of the subject invention in whichan automatic antenna tuner remains coupled to the antenna and isoperated at full power, the values of the inductors and capacitors inthe automatic antenna tuner being utilized in combination with thecoaxial cable length and velocity factor to directly calculate theantenna input impedance from a point remote from the feed point of theantenna;

FIG. 4 is a diagrammatic illustration of the microprocessor within theautomatic antenna tuner of FIG. 3, wherein once the inductor values andthe capacitor values are arrived at upon an SWR equaling 1:1, theantenna input feed point impedance is directly calculated;

FIG. 5 is a detailed block diagram of the subject system indicating thatwhen a transmitter sends RF power to an antenna through an L impedancematching network, and the SWR measuring circuit monitors the SWR of theL impedance matching network, a microprocessor that tries combinationsof the inductors and capacitors in the L network using an algorithm thatconverges toward SWR=1 and monitors the SWR until a combination ofinductors and capacitors achieves a suitably low SWR, ideally 1:1, andthe frequency is measured, antenna input impedance is calculated fromthe values of the L network and is read out to a display;

FIGS. 6A and 6B are schematic diagrams showing two cases for a typical Limpedance matching network in which the capacitor to ground is connectedfirst to the transmitter side of the inductor and then to the antennaside; and,

FIG. 7 is a schematic diagram of an automatic antenna tuner in whichinductors and capacitors are switched into the circuit to achieve an SWRof 1:1, with the antenna input impedance calculated from the state ofthe switches when the minimum SWR is achieved.

DETAILED DESCRIPTION

Referring now to FIG. 1, in the prior art, in order to obtain theantenna input impedance at the feed point of an antenna which is remotefrom the tuner, in Step 1 of this process, antenna 10 is coupled to atuner 12 which has an SWR meter 14 indicating the SWR measured at thetuner. Antenna 10 is coupled to tuner 12 at antenna input terminals 16,with a transmitter 18 coupled to the transmitter input 20 of tuner 12.

In Step 2 of this prior art process, antenna 10 is disconnected from theantenna input terminal 16 of tuner 12 and a series RC and/or an RLcircuit having a variable capacitor 22 and/or a variable inductor (notshown) and a variable resistor 24 is connected in series across theantenna input terminals 16 of tuner 12. At the same time, transmitter 18is disconnected from tuner 12 and an antenna analyzer 26 is connected tothe transmitter input terminals of tuner 12.

The capacitive element 22 and/or inductive element (not shown) andresistive element 24 are adjusted until a minimum SWR, ideally 1:1, isachieved. At this point the impedance of the resistive and capacitiveand/or inductive elements is equal to the antenna feed point impedance.

Referring to FIG. 2, in a second method as a first step tuner 12 isconnected to antenna 10 and the SWR is measured by SWR meter 14 when anantenna analyzer 26 is substituted for transmitter 18. Once the antennatuner is tuned to achieve a 1:1 SWR utilizing an antenna analyzer whichis tuned to the intended transmitter frequency, and as illustrated inStep 2, antenna 10 is removed from tuner 12 and the antenna analyzer 26is coupled to the antenna input terminal 16 of tuner 12 which has beentuned to achieve an SWR 1:1 at the particular transmitter frequency ofinterest. In Step 2, antenna analyzer 26 measures the impedance of thetuned tuner when a 50 ohm resistor 30 is coupled across the transmitterinput terminal of the tuner.

It can be shown that the complex conjugate of the measured impedance ofthe tuned tuner as established by antenna analyzer 26 is the antennafeed point input impedance.

It will be appreciated that in both of these prior art methods toascertain antenna input impedance from a remote location, an antennaanalyzer is utilized. As mentioned hereinbefore, antenna analyzersoperate in the milliwatt range and the results are easily swamped bylocal RF radiation which corrupts the reading. In either of these casesthe transmitter is disconnected, as is the antenna, and these techniquesare operated at anything but full power due to the use of the antennaanalyzers. Moreover, the prior art system of FIG. 1 could not beutilized at full power due to the power restrictions involved in thecapacitive and resistive components. Thus the prior art systems of FIGS.1 and 2 are cumbersome and error-prone.

Referring to FIG. 3, in the subject system an automatic antenna tuner 40is coupled to antenna 10 via a coaxial cable 42 having a length 44 fromantenna feedpoint 46 to the antenna input to tuner 40, here shown at 48.This antenna tuner may be one of a number of automatic antenna tunersthat operate by switching in and out inductors and capacitors until aminimum SWR is achieved. Here automatic antenna tuner 40 is providedwith a Crosspoint SWR meter 50, with the antenna tuner driven by a fullpower transmitter 52 at a given frequency.

As will be discussed, the antenna feedpoint impedance illustrated bydisplay 54 is calculated by the values of the capacitive elements andinductive elements of the antenna tuner when the antenna tuner achievesa minimum SWR. In order to calculate antenna feed point impedancecoaxial cable length 44 is entered into the antenna tuner as illustratedby up/down switches 55, whereas the coaxial cable velocity factor 56 isentered by up/down switches 58.

In operation, the antenna tuner is made to tune by the depression of atune switch 60 which causes the antenna tuner to cycle through all ofits capacitive and inductive settings to switch in and out therespective capacitors and inductors to achieve a minimum SWR. Theantenna tuner automatically senses the frequency of transmitter 52 andthis transmitter frequency along with the coaxial cable length andvelocity factor is utilized in conjunction with the values of thecomponents switched into the circuit to achieve the minimum SWR toderive the antenna feedpoint impedance displayed a display 54.

In some antenna tuners it is necessary to reduce the power coupled tothe antenna tuner to be able to initially set up the antenna tuner fortuning. Once tune has been established, full power may be applied to theantenna tuner so that feedpoint impedance is not corrupted by local highpower RF signals and so that the system can be run at full operationalpower.

Referring to FIG. 4, what is depicted is the operation of themicroprocessor utilized in antenna tuner 40 of FIG. 3. Heremicroprocessor 60 is utilized to switch in and out the inductive andcapacitive values of the antenna tuner until a minimum SWR is achieved,preferably 1:1. It is then the purpose of the microprocessor tocalculate antenna feed point impedance utilizing the entered coaxialcable length and velocity factor, with the input frequency of alreadyhaving been internally measured by the automatic antenna tuner.

The formula by which the antenna input impedance is measured in oneembodiment is now described:

The complex conjugate of tuner input impedance is_Z_(in) andZ_(in)R_(in)+jX_(in)

-   -   where R_(in)=RX_(c) ²/(R²+X_(c) ²) for R less than 50 ohms and        X_(in)=X_(L)−[R²X_(c)/(R²+X_(c) ²)], where R is the transmitter        output impedance and Z_(in) is the tuner input impedance;    -   and for R greater than 50 ohms:    -   Z_(in)=R_(in)+jX_(in) where    -   R_(in)=RX_(c) ²/[R²+(X_(L)−X_(c))²] the resistance of the        antenna    -   X_(in)=−X_(c)[X_(L)(X_(L)−X_(c))+R²]/[R²+(X_(L)−X_(c))²] the        reactance of the antenna and    -   Z_(ant)=R_(in)−jX_(in)

Referring now to FIG. 5, from a schematic point of view, the systeminvolves a transmitter 70, an SWR measuring circuit 72, and L impedancematching network 74 and an antenna 76, the impedance of which is to bemeasured. The frequency at which transmitter 70 is operated is detectedby a microprocessor controller board 78, with the output of the SWRmeasuring circuit coupled to the microprocessor controller board asillustrated at 80, and with the microprocessors controller boardcontrolling the L impedance matching network as illustrated by arrow 82.When the SWR has been measured and the L impedance matching network haslocked up to provide the minimum SWR, the result is displayed by adigital LCD/LED analog readout 80 which reads R and X and the sign ofthe impedance, as well as SWR frequency and other parameters.

As indicated in this drawing, the transmitter couples RF power to theantenna through an L impedance matching network. The SWR measuringcircuit monitors SWR associated with the L impedance matching network.The microprocessor board tries all combinations of inductors andcapacitors in the L network and monitors the SWR until a combination ofinductive values and capacitive values is found where the SWR is low,ideally 1:1. The frequency of transmitter 70 is also measured, with thelength of the transmission line and its velocity factor entered into tocalculate antenna input impedance.

Referring to FIGS. 6A and 6B, there are two cases for a typical Limpedance matching network. It will be noticed that the capacitorbetween the inductor and ground in once case is to the transmitter sideof the inductor and is the preferred position where the load impedanceis for instance less than 50 ohms. Where the load impedance is greaterthan 50 ohms, the preferred position of the capacitor is to the antennaside of the inductor. In operation, the automatic antenna tuner switchesthe position of the capacitor from one side of the inductor to the otherand measures that position which gives the lowest SWR. When this isestablished, the component values for inductance and capacitance whichled to the lowest SWR are detected and the antenna input impedance iscalculated therefrom. Once having established the input impedance to theantenna tuner when the antenna tuner has tuned, the complex conjugateyields the input impedance of the antenna which is corrected for thelength of the cable and velocity factor.

Referring to FIG. 7, a typical automatic antenna tuner includes atransmitter input 90 which is coupled to an antenna through a series ofinductors L3-L10 here illustrated at 92 which are switched into and outof the circuit is illustrated by switches K-3-K 10 so that either theinductors are shorted or are left in series in the circuit. A capacitorbank C 73-C 84 is connected to the antenna or transmitter side via K19to ground as illustrated in which a series of capacitors 94 is connectedto ground via switches K 11-K18. Note that switches K-3-K 10 and K 11-K18 are controlled by the aforementioned microcontroller such that theassociated components are switched into an out of the circuit until themeasured SWR is at a minimum. Also shown is a switch K-1 to connect thetuner to one of a number of antenna connectors.

The state of all the switches K-3-K 18 is used by the microprocessor tocompute antenna input impedance as illustrated at 98 which is thendisplayed as illustrated at 100. The values of the inductors andcapacitors are as listed in this figure.

It is noted that there are number of the parallel resistors coupledbetween the coaxial transmitter input 90 and the bank of inductors whichmay be shorted out as illustrated by switch K1. Note also that thecircuit coupled to transformer T1 is used to determine the SWR, amongstother things.

It will therefore be seen that a standard L impedance matching networkantenna tuner, when a sufficiently low SWR has been achieved, can beutilized to directly compute antenna input impedance remote from theantenna, knowing the frequency at which the system is operating plus thelength of transmission line and its velocity factor. This providesconsiderable convenience to the radio operator to be able to ascertain,at a glance, the proper operation of an antenna which is remote from thestation transmitter and to do so without disconnecting the antenna orutilizing a low power antenna analyzer.

While the present invention has been described in connection with thepreferred embodiments of the various figures, it is to be understoodthat other similar embodiments may be used or modifications or additionsmay be made to the described embodiment for performing the same functionof the present invention without deviating therefrom. Therefore, thepresent invention should not be limited to any single embodiment, butrather construed in breadth and scope in accordance with the recitationof the appended claims.

What is claimed is:
 1. Apparatus for remotely measuring feed pointimpedance of an antenna, comprising: an automatic antenna tuner having anumber of components that are switched into the tuner circuit to achievea minimum SWR; a processor for calculating antenna feed point impedancebased on the values of the components that are utilized to achieve saidminimum SWR; and, a display coupled to said processor for displayingsaid calculated antenna feed point impedance.
 2. The apparatus of claim1, wherein the values of said components at minimum SWR are derived fromswitch states of switches utilized to switch said components into thetuner circuit when said minimum SWR has been achieved.
 3. The apparatusof claim 1, wherein said remotely sensed antenna feed point impedance isgiven by the complex conjugate of the tuner input impedance at the timeof tune.
 4. The apparatus of claim 3, wherein for an L network, thecomplex conjugate of tuner input impedance is Z_(ant)=R_(in)−jX_(in),where for R less than 50 ohms Z_(in)=R_(in)+jX_(in) where R_(in)=RX_(c)²/(R²+X_(c) ²) and X_(in)=X_(L)−[R²X_(C)/(R²+X_(C) ²)], where R is thetransmitter output impedance and Z_(in) is the tuner input impedence;and for R greater than 50 ohms Z_(in)=R_(in)+jX_(in) where R_(in)=RX_(c)²/[R²+(X_(L)−X_(c))²] the resistance of the antennaX_(in)=−X_(c)[X_(L)(X_(L)−X_(c))+R²]/[R²+(X_(L)−X_(c))²] the reactanceof the antenna and Z_(ant)=R_(in)−jX_(in)
 5. The apparatus of claim 3,wherein X_(c) is the combined capacitance of all of the capacitiveelements that are switched in to achieve said minimum SWR wherein theinductance X_(L) is the combined inductance of all inductive elementsthat are switched into it she said minimum SWR.
 6. A method for remotelysensing antenna input impedance of an antenna coupled to a transmitterutilizing a feedline, comprising the steps of: utilizing an automaticantenna tuner to tune said antenna to said transmitter; and, detectingthe antenna input impedance utilizing characteristics of the antennatuner when the antenna tuner has tuned to a minimum SWR.
 7. The methodof claim 6, wherein said automatic antenna tuner includes capacitive andinductive elements that are switched in to achieve the minimum SWR andwherein said detecting step includes utilizing the values of theswitched in elements to compute the antenna input impedance.
 8. Themethod of claim 7, wherein the detecting step includes measuring thelength of the feedline between the automatic antenna tuner and theantenna feed point.
 9. The method of claim 8, wherein the detecting stepincludes ascertaining the velocity factor of the transmission line. 10.The method of claim 9, wherein the detecting step includes ascertainingthe frequency at which the antenna is driven.
 11. The method of claim 6,wherein the automatic antenna tuner includes a number of capacitive andinductive components that are switched into the antenna tuner circuitutilizing switches, and wherein the detecting step includes ascertainingthe switch states of the switches when minimum SWR is achieved.
 12. Themethod of claim 6, and further including displaying the calculatedantenna input impedance.
 13. The method of claim 12, wherein thecalculated antenna input impedance is displayed at the automatic antennatuner, whereby the antenna input impedance is detected and displayedremote from the antenna feed point.
 14. The method of claim 6, whereinthe measurement of antenna feed point input impedance is performed atfull transmitter power.
 15. The method of claim 6, wherein themeasurement of antenna feed point input impedance is performed withoutusing a low power antenna analyzer.
 16. A method for remotelydetermining the antenna input impedance of an antenna by utilizing anautomatic antenna tuner and arriving in the antenna input impedance fromcharacteristics of the automatic antenna tuner without utilizing antennaanalyzer.
 17. The method of claim 16, wherein the determination ofantenna input impedance is performed at full transmitter power so as tominimize the effects of any local high power RF energy that could swampthe measurement and to detect the effects of transmitter power on theantenna.